In many types of delay lines, phase shifters and similar circuits, it is necessary to be able to vary the value of a capacitance in response to an input control voltage. This is typically done by use of what is known as a varactor diode. However, such diodes possess numerous shortcomings. One such shortcoming is that the change is capacitance with respect to a change in control voltage is linear over a very small range.
The voltage-controlled capacitor circuit described in the above-noted parent application has a considerably wider range over which the change in capacitance is linear with respect to the control voltage. One method of increasing the range even further is to incorporate a precompensation circuit, sometimes referred to as a predistortion circuit, between the control voltage source and the control voltage input of the voltage-controlled capacitor circuit. Such precompensation circuits alter the magnitude of the control voltage applied to the capacitor circuit to compensate for the non-linearities in the voltage-controlled circuit.
One precompensation network utilizes a differential amplifier which includes a pair of transistors having their emitters connected to a constant current source. A pair of diodes are connected in the collector circuits of the transistors with the differential output of the amplifier being coupled to the input of the voltage-controlled capacitor circuit.
The diode loads of the precompensation circuit produce a non-linear differential output voltage which is generally logarithmic in nature. This logarithmic output tends to compensate for the exponential-type non-linearities which are present in the voltage-controlled capacitor circuit. Thus, the control voltage/capacitance relationship is linear over an increased range.
A major disadvantage of precompensation circuits utilizing diode pairs is that the diodes must be closely matched. Preferably, the diodes are formed from die taken from a common wafer, preferably adjacent die. Thus, the cost of the precompensation circuit is greatly increased.
The subject precompensation circuit overcomes the previously-noted disadvantages in that distortion correction is accomplished without the use of matched diodes. Additional advantages of the disclosed circuit will become apparent to those skilled in the art after having read the following Best Mode For Carrying Out the Invention together with the drawings.